Enter existing user namespace if present

[codido]

When executing an additional process in a container, all namespaces are
entered but the user namespace. As a result, the process may be
executed as the host's root user. This has both functionality and
security implications.

Fix this by adding the missing user namespace to the array of
namespaces.

[hqhq]

I think we already handle user ns in golang implementation, and also, FYI #105 is modifying some mechanism.

[codido]

#105 indeed fixes the issue I was seeing. If it's going to be merged, I can close this pull request.

[dqminh]

i haven't look at #105 for a while :( I will try to give it some time this week. Generally #105 approach is the way to go i think, as init processes should also be able to share namespaces, not necessarily only exec-in processes.

[mrunalp]

Thanks for the PR @GitIdo. However, I think that we should close this in favor of #105